Semiconductor laser diodes have numerous advantages. One advantage is the small size of the laser diodes. For example, an active region of a laser diode has a width that is typically a submicron to a few microns, a height that is usually no more than a fraction of a millimeter, and a length that is typically less than about a millimeter. Internal reflective surfaces, which produce emission in one direction, are formed by cleaving the substrate from which the laser diodes are produced and, thus, have high mechanical stability.
High efficiencies are possible with semiconductor laser diodes with some having external quantum efficiencies near 70%. Semiconductor laser diodes produce radiation at wavelengths from about 20 to about 0.7 microns depending on the semiconductor alloy that is used. For example, laser diodes manufactured from gallium arsenide with aluminum doping (“AlGaAs”) emit radiation at approximately 0.8 microns (˜800 nm), which is near the absorption spectrum of common solid state laser rods and slabs manufactured from Neodymium-doped, Yttrium-Aluminum Garnet (“Nd:YAG”), and other crystals and glasses. Thus, semiconductor laser diodes can be used as an optical pumping source for larger, solid state laser systems.
Universal utilization of semiconductor laser diodes has been restricted by thermally related problems. These problems are associated with the large heat dissipation per unit area of the laser diodes resulting in elevated junction temperatures and stresses induced by thermal cycling. Laser diode efficiency and the service life of the laser diode are decreased as the operating temperature in the junction of the laser diode increases.
Furthermore, the emitted wavelength of a laser diode is a function of its junction temperature. Thus, when a specific output wavelength is desired, maintaining a constant junction temperature is essential. For example, AlGaAs laser diodes that are used to pump an Nd:YAG rod or slab emit radiation at about 808 nm because this is the wavelength at which optimum energy absorption exists in the Nd:YAG. However, for every 3.5° C. to 4.0° C. deviation in the junction temperature of the AlGaAs laser diode, the wavelength shifts 1 nm. Accordingly, controlling the junction temperature and, thus, properly dissipating the heat is critical.
When solid state laser rods or slabs are pumped by laser diodes, dissipation of the heat becomes more problematic because it becomes necessary to densely pack multiple individual diodes into arrays that generate the required amounts of input power for the larger, solid state laser rod or slab. However, when the packing density of the individual laser diodes is increased, the space available for extraction of heat from the individual laser diodes decreases. This aggravates the problem of heat extraction from the arrays of densely packed individual diodes.
Currently, laser emitters may be in a laser diode bar having strips of laser diodes or single-emitter laser diode chips. A laser diode bar consists of a linear array of physical connected single emitter laser diodes. The laser diode bar configuration requires the emitters to be operated in parallel and the individual emitters cannot be electrically isolated because the emitters are physically connected. A number of laser diode bars are arranged axially around a laser medium. However, since a laser diode bar is of a certain width, the ability to place such diodes in optimal proximity to a laser medium is limited and therefore results in poor angular pumping uniformity and induces stress birefringence thus limiting gain. For example, five laser bars each having 25 laser diodes may be arranged in a pentagon shape, lengthwise, around a laser rod. The pitch or number of bars that may be placed around the laser rod is also limited. The bar configuration also has several other problems. For example, the failure of one emitter on a bar may cause total failure because the emitters are coupled in series together.
An alternative to the laser diode bars are much smaller single emitter laser diodes arranged transversely around the perimeter of the laser medium. The use of individual emitters may be implemented on a thinner substrate because such emitters may be attached to the surface of a layer. Individual layers may allow integrated protection circuitry and may be capable of hard soldering to other components. However, the use of single emitters requires additional support elements which inhibit compact arrays due to the increased thickness around the laser medium. Further, single emitters require additional electrical components because the emitters must be physically separated.
Thus, it would be desirable to have a pump source solution that combines the compactness of the laser bar arrays and the effectiveness of micro-channel cooling with the robustness of single emitters. The use of single emitters in a compact arrangement to allow a ring shaped arrangement of laser diodes transversely around a laser medium would also be desirable.